VBA si #NA alors valeur precedente.

Sub bouton1_clic()
Dim nbligne As Integer 'Long si plus de 32 767 lignes

With Sheets("tableau")
    nblig = .Cells.SpecialCells(xlCellTypeLastCell).Row
    For i = 6 To nblig
        .Cells(i, 17).Formula = IIf(IsError(Application.VLookup(Sheets("tableau").Range("A" & i), Sheets("extraction").Range("A1:J" & nblig), 10, False)), .Cells(i, 17), Application.VLookup(Sheets("tableau").Range("A" & i), Sheets("extraction").Range("A1:J" & nblig), 10, False))
    Next i
End With
MsgBox "données initialisées"
End Sub

Sub Bouton1_Clic()
Dim T As Worksheet
Dim E As Worksheet
Dim DL As Integer
Dim nbligne As Integer

Set T = Worksheets("Tableau")
Set E = Worksheets("extraction")
DL = E.Range("B" & Application.Rows.Count).End(xlUp).Row
Nblig = T.Cells.SpecialCells(xlCellTypeLastCell).Row
For i = 2 To Nblig
   T.Cells(i, 3).Value = IIf(IsError(Application.VLookup(T.Range("B" & i), E.Range("B1:C" & DL), 2, False)), T.Cells(i, 3), Application.VLookup(T.Range("B" & i), E.Range("B1:C" & DL), 2, False))
Next i
MsgBox "Données initialisées"
End Sub